Continued worldwide demand for petroleum products, combined with a high level of prices for petroleum and products recovered therefrom, has sustained interest in the sources of hydrocarbons which are less accessible than crude oil of the Middle East and other countries. Such hydrocarbonaceous deposits range from heavy oil to tar sands and to oil shale, found in western Canada and in the western United States. Depending on the type and depth of the deposit, recovery techniques range from steam injection to in-situ combustion to mining.
For heavy oils in the gravity range of 10 to 20 degrees API, steam injection has been a widely-applied method for oil recovery. Problems arise, however, when one attempts to apply the process to heavy oil reservoirs with very low transmissibility. In such cases, because of the unfavorable mobility ratios, steam channelling and gravity override often result in early steam breakthrough and leave a large portion of the reservoir unswept. The key to a successful steam flooding lies in striking a good balance between the rate of displacement and the rate of heat transfer which lowers the oil viscosity to a more favorable mobility ratio.
A more particular problem is presented when the oil-bearing formation contains vertical fractures or other conduits which are narrow in lateral extent. Where these conduits link injection wells with production wells, injected steam flows quickly to the production wells resulting in high water-oil ratios and low oil recovery. The problem is resolved with the instant invention.